Optical imaging systems are used in many fields in technology and research, ever more stringent requirements being made of said imaging systems with regard to their imaging quality. One example is the microlithographic production of semiconductor components and other finely structured components, wherein structures in the submicron range can be produced with the aid of optical imaging systems in the form of projection lenses. Such imaging systems have a complex optical construction with a multiplicity of optical elements, which generally makes it impossible to derive the real optical properties from theoretical calculations. Therefore, the optical properties of imaging systems have to be measured reliably.
Interferometric measurement methods are often used for this purpose. A wavefront detection device which operates in the manner of a shearing interferometer and which enables a fast, wholly accurate measurement of extremely high resolution photolithographic projection lenses is described in WO 2001/063233 A1 (corresponding to US 2002/0001088 A1). In said device, a mask is arranged in the object plane of the imaging system to be measured. The mask comprises a rigid, transparent structure carrier produced from quartz glass, for example, on which a two-dimensional object pattern is applied, for example by suitable coating with chromium. The mask is illuminated with incoherent light for measurement purposes. A reference pattern designed as a diffraction grating is arranged in the image plane of the imaging system. The coherence of the radiation passing through the projection lens is determined by the object pattern. The superposition of the waves generated by diffraction at the diffraction grating gives rise to a superposition pattern in the form of an interferogram, which is detected with the aid of a suitable (spatially resolving) detector and is subsequently evaluated. In order to be able to calculate a two-dimensional phase distribution from the interferograms, a plurality of interferograms with different phase angles are detected.
In a microlithographic projection exposure method, use is made of a mask (reticle) that bears the pattern of a structure to be imaged, e.g. a line pattern of a layer of a semiconductor component. The pattern is positioned in a projection exposure apparatus between an illumination system and a projection lens in the region of the object plane of the projection lens and is illuminated with an illumination radiation provided by the illumination system. The radiation changed by the pattern passes as projection radiation through the projection lens, which images the pattern onto the substrate which is to be exposed and is coated with a radiation-sensitive layer and whose surface lies in the image plane of the projection lens, said image plane being optically conjugate with respect to the object plane.
In order to be able to produce ever finer structures, in recent years optical imaging systems have been developed which operate with moderate numerical apertures and obtain high resolution capabilities substantially with the short wavelength of the electromagnetic radiation used from the extreme ultraviolet range (EUV), in particular with operating wavelengths in the range of between 5 nm and 30 nm. Radiation from the extreme ultraviolet range cannot be focused or guided with the aid of refractive optical elements, since the short wavelengths are absorbed by the known optical materials that are transparent at higher wavelengths. Therefore, mirror systems are used for EUV lithography. In the field of EUV microlithography, too, endeavours are made to further increase the resolution capability of the systems used by developing projection systems having an ever higher image-side numerical aperture NA, in order to be able to produce ever finer structures. For a given imaging scale β, the object-side numerical aperture NAO thus increases as well.
For higher-aperture EUV systems, narrowband masks pose a challenge because their reflectivity capability decreases greatly at larger angles of incidence of the radiation. Therefore, it has already been proposed to use greater reductions instead of the customary reducing imaging scale of 1:4 (|ß|=0.25) for lithographic-optical systems. By way of example, an imaging scale of 1:8 (|ß|=0.125) instead of 1:4 (|ß|=0.25) halves the object-side numerical aperture NAO and thus also the angles of incidence of the illumination radiation at the mask by half. However, this imaging scale (for the same mask size) reduces the size of the exposed field and thus the throughput.
It has also already been recognized that when the object-side numerical aperture is increased, the object-side principal ray angle must be increased, which can lead to shading effects by the absorber structure of the mask and to problems with the layer transmission. In particular, severe apodization effects can occur owing to the reticle coating (cf. e.g. WO 2011/120821 A1).
WO 2012/034995 A2 proposes for this reason, inter alia, designing an EUV projection lens as an anamorphic projection lens. An anamorphic projection lens is characterized in that a first imaging scale in a first direction deviates from a second imaging scale in a second direction perpendicular to the first direction. The deviation lies significantly outside deviations possibly caused by manufacturing tolerances. An anamorphic projection lens enables e.g. a complete illumination of an image plane with a large object-side numerical aperture in the first direction, without the extent of the reticle to be imaged in said first direction having to be increased and without the throughput of the projection exposure apparatus being reduced. Furthermore, in comparison with systems having a uniform imaging scale in both directions, a reduction of the losses of imaging quality that are caused by the oblique incidence of the illumination light can also be obtained.